1. Field of the Invention
The present invention relates to a lithium-ion battery, and in particular relates to safety and life characteristics of a lithium-ion battery where a winding group having a positive electrode, a negative electrode and at least one separator, and a connecting portion for connecting to respective terminals from the winding group are accommodated in a battery container, and which is provided with an inner pressure-reducing mechanism for discharging gas according to an increase in inner pressure inside the battery container.
2. Description of the Related Art
Because a lithium-ion secondary battery or cell has high energy density as its merit, it is being used as a power supply source for portable equipment mainly as a VTR camera, a note type computer, a portable telephone and the like. In an interior of a cylindrical lithium-ion second battery, a winding type structure is formed where a positive electrode member and a negative electrode member, each comprising a strip-shaped metal foil applied with active material, are wound spirally in section such that they are prevented from contacting each other by a separator interposed therebetween, thereby forming a winding group or a winding body. The winding group is accommodated in a cylindrical battery can serving as a battery container, and then the can is sealed after pouring electrolytic solution into the can.
An ordinary cylindrical lithium-ion battery has an external dimension of a diameter of 18 mm and a height of 65 mm, which is called 18650 type, and it is widely spread as a small-sized lithium-ion battery for a civilian use. Lithium cobaltate, having high capacity. and long life characteristics, is mainly used as positive electrode active material for the 18650 type lithium-ion battery, and battery capacity thereof is approximately 1.3 Ah to 1.7 Ah and battery power (output) thereof is about 10 W or so.
Meanwhile, in order to cope with the environmental problems in the automotive industry, development of electric vehicles (EVs) whose power sources are confined completely to batteries so that there is no gas exhausting and development of hybrid electric vehicles (HEVs) where both internal combustion engines and batteries are used as their power sources have been facilitated and some of them have reached a practical stage. Secondary batteries for the EVs and HEVs are required to have high power and high-energy characteristics. Attention to the lithium-ion batteries is being paid as secondary batteries that can meet this requirement. In order to obtain high capacity and high power secondary batteries, improvements in dimensions for electrodes have been devised to date. For example, Japanese Patent No. 2701347 discloses the optimal values of thickness of composing material layers for the positive electrode and negative electrode members.
However, in the case of the lithium-ion batteries, as the power becomes higher, the safety tends to be regarded as important. Specifically, in such high capacity and high power batteries used as power sources for the EVs and HEVs, the safety is not secured necessarily in the range disclosed in the Japanese Patent No. 2701347. Further, since the high capacity and power batteries are required to allow large current charging and large current discharging, it is difficult to provide such batteries with a current cut-off mechanism (a shutdown switch of a kind) that actuates in response to an increase in inner pressure inside the batteries at an abnormal time, which is generally employed in the 18650 type lithium-ion batteries.
In a case in which the EV or HEV carries persons or occupants, it is at least one necessary and extremely important characteristic to secure safety of a battery itself at a time of overcharging which may occur when a charging control system has been broken down, at a time of battery crush, at a time of foreign matter thrusting, at a time of external short-circuiting or the like which may be encountered at an accidental collision of the vehicle equipped with the battery. The term xe2x80x9csafety of a batteryxe2x80x9d used herein means that the behavior of a battery fallen into such abnormal states is not only prevented from insuring vehicle occupants including a driver physically but also it is suppressed from damaging the vehicle body as least as possible.
In general, carbon material used as negative electrode active material for the lithium-ion battery is in a state in which lithium ions are completely discharged, namely, the carbon material is in a discharged state. Accordingly, active material of a discharged state, for example, lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4) or the like is also used for a positive electrode. However, since these active materials for the positive electrode have not sufficient electroconductivity, they are mixed, together with binder, for use with electroconductive powder serving as electroconductive material such as graphite powder, carbon black powder or the like which is stable and inexpensive.
The lithium manganate having the spinel structure as a crystal structure has a feature that its thermal stability is superior to that of the lithium cobaltate or lithium nickelate. For this reason, a battery where the lithium manganate is used for positive electrode active material is suitable as a high safety battery for a large-sized lithium-ion battery for power storage, the EV or HEV, or the like. However, the crystal of the lithium manganate expands/shrinks according to insertion/department of lithium ions (or occlusion/release thereof) due to charging/discharging. Then, when the charging/discharging accompanied with expansion/shrinking is repeated, the electroconductivity required for the positive electrode deteriorates and the discharging capacity decreases. Further, in the positive electrode where the lithium manganate is used as the active material, its manganese component dissolves into non-aqueous electrolytic solution, irrespective of a charging/discharging state. The dissolution of the manganese component causes dropping in a charging/discharging cycle life and/or declining in preserving characteristics.
For improving the charging/discharging cycle characteristic in the lithium-ion battery, Japanese Patent Application Laid-Open (JP-A) No. 10-182160 discloses a technique for manufacturing lithium manganate with a high crystalline property by improving the structural conditions of the lithium manganate, or by doping additive or the like. JP-A No. 10-182157 also discloses a technique for doping elements of different kinds in the crystalline structure of the lithium manganate. However, a sufficient cycle life has not attained to date.
In view of the above circumstances, an object of the present invention is to provide a lithium-ion battery which retains high safety while having high capacity and high power.
Also, another object of the invention is to improve a cycle life of the lithium-ion battery using lithium-manganese complex oxide for a positive electrode.
In order to achieve the above first object, a first aspect of the present invention is a lithium-ion battery where a winding group, having a positive electrode, a negative electrode and at least one separator, and a connecting portion for connecting to respective terminals from the winding group are accommodated in a battery container, and which is provided with an inner pressure-reducing mechanism for discharging gas according to an increase in inner pressure inside the battery container, wherein the positive electrode comprises a collector whose both surfaces are applied with composing material including lithium-manganese complex oxide, the thickness of the composing material on the both surfaces of the collector is at least 210 xcexcm, and the amount of the active material per one surface of the collector is at least 240 g/m2.
According to the aspect, in order to secure a lithium-ion battery with high capacity and high power, the positive electrode comprises the collector whose both surfaces are applied with composing material including lithium-manganese complex oxides. In the lithium-ion battery, when the abnormal state described above occurs, gas is generated due to reaction between electrolytic solution and the composing material including the lithium-manganese complex oxide while maintaining a large current charging state or a large current discharging state, and thereby inner pressure inside the battery container increases in some cases. In order to prevent the increase in the inner pressure, the lithium-ion battery generally has an inner pressure-reducing mechanism such as a safety valve, a rupturable valve or the like for discharging gas out of the container at predetermined inner pressure. In the lithium battery provided with such a mechanism, in a case in which the thickness of the composing material on the both surfaces of the collector is set to at least 210 xcexcm and the amount of the active material per one surface of the collector is set to at least 240 g/m2, the gas is much gently discharged without any violent discharging from the mechanism. Accordingly, a battery superior in safety can be obtained.
In this aspect, in a case in which a compounding ratio of the lithium-manganese complex oxide is set to at least 80 weight %, a lithium-ion battery with high capacity and high safety can be realized since the discharging capacity can be prevented from lowering. Also, it is preferable that the negative electrode comprises a second collector whose both surfaces are applied with second composing material including carbon material, and the thickness of the second composing material on the both surfaces of the second collector is set to at least 130 xcexcm.
The present inventors have studied eagerly a case in which the lithium manganate is used for the positive electrode, and as a result of it, realized that, when the lithium manganate is used for the positive electrode, the deterioration of the negative electrode is larger than that in a case of using lithium cobaltate for the positive electrode. The inventors have also found the causes: Since the volume of the lithium manganate reduces at a time of charging, the pressurizing force of the winding group drops. As this result, collapse in an electroconductive network in the composing material of the negative electrode is brought about. Therefore, in order to achieve the another object, according to a second aspect of the present invention, a lithium-ion battery where a winding group having a positive electrode, a negative electrode and at least one separator, and a connecting portion for connecting to respective terminals from the winding group are accommodated in a battery container, and which is provided with an inner pressure-reducing mechanism for discharging gas according to an increase in inner pressure inside the battery container, wherein the second composing material contains fiber-shaped or needle-shaped conductive material.
According to the second aspect, since the conductive material retains the electroconductive network in the composing material of the negative electrode, the cycle characteristic can be improved. In this aspect, it is preferable that the conductive material is carbon fibers manufactured by a vapor-phase method or conductive ceramic fibers, and it is more preferable that an average length of the fibers is at least 5 xcexcm and an average diameter of the fibers is at most 5 xcexcm.
The present invention will become more obvious by referring to the following preferable embodiments.